During the course of normal embryonic development the fetal testis produces a substance which causes regression of the Mullerian ducts. The existance of this substance has been appreciated for 25 years but technical problems have limited the intensive study of its specific nature. We hope to surmount some of these technical difficulties and to produce the substance in large enough quantity to allow us 1) to isolate and purify the Mullerian Inhibiting substance 2) to demonstrate its mode of action at the cellular level, 3) to elucidate its role in the endocrine control of normal sex differentiation, and 4) to determine its potential role as a regressor of tumors of Mullerian duct origin. The techniques to be used in this effort will involve the organ culture assay of Picon and Josso, and a tissue culture assay with Mullerian duct cells as the target cells. The active substance will be harvested from the testes of hypophysectomized rats and from the media of a cell line capable of elaborating Mullerian Inhibiting Substance in vitro. Human tumors of Mullerian duct origin will be passaged in tissue culture to study the potential role of the Mullerian Inhibiting Substance as a tumor regressor by demonstrating a cytopathic effect against target tumor cells. The role of pituitary gonadotropins in the control of Mullerian Inhibiting Substance production will be studied both in vivo and in vitro.